Conventionally, an unfixed toner image transferred to a recording medium is required to be fixed permanently by an image forming apparatus such as a copying machine and a printer, and Japanese unexamined patent publications Sho 5269337 (1977), 60-151677 (1985), 60-151681 (1985), 62-14675 (1987), Japanese unexamined utility model publications Sho 60-104852 (1985) and Hei 2-30961 (1990) describe fixing devices, wherein high quality fixing of even a color image having a large amount of unfixed toner is carried out by passing a recording medium having a transferred unfixed toner image thereon through a long contact portion formed between the fixing roller and an endless belt contacting the roller under pressure, and applying heat and pressure to the unfixed toner image for a long time.
Although the above described fixing device carries out high quality fixing of even a color image having a large amount of unfixed toner, especially when fixing a color image having a large amount of unfixed toner, because of the tendency of the toner to act as an adhesive, and for the recording medium to stick to the fixing roller, a stripper claw is required to positively strip the recording medium from the fixing roller . This stripper claw, however, has a tendency to damage the image fixed on the recording medium and impair the image quality.
The present inventors have proposed the following fixing device to solve this problem in Japanese patent application No. Hei 3-252097.
This is, as shown in FIG. 7, what is called a belt contact fuser for a fixing device using a thermal fixing roller, which fixes unfixed toner image 101 transferred to a transfer medium 100 and comprises a thermal fixing roller 103 with an elastic body 102 having a thickness of at least 0.5 mm applied thereon, a heat resistant belt 108 supported by a plurality of support rollers 104 to 107, and wherein the heat resistant belt 108 is made to contact the fixing roller 103 to make a contact portion extending over a certain angular range. A pressure roller 107 is disposed inside the heat resistant belt 108 at the exit point of the contact portion and it bears against the thermal fixing roller 103 under pressure through the heat resistant belt 108, such that distortion is caused in the elastic body 102 of the thermal fixing roller 103.
This belt contact fixing device can be applied not only to a monochrome but also to a color copying machine, in which distortion is caused in the elastic body 102 of the thermal fixing roller 103 by making the pressure roller 107 contact the thermal fixing roller 103 through the heat resistant belt 108 and the distortion allows the recording medium to strip from the thermal fixing roller 103 by its own stiffness. With this fixing device, a stripping means such as a stripper claw is not necessary at the exit point of the contact portion even in fixing a color image having a large amount of toner and transferred to a thin recording medium having a poor stiffness. This behavior, wherein the recording medium is stripped without a stripping means at the exit point of the contact portion, is called self-stripping.
The above described prior art, however, has a problem. This prior art comprises a heat resistant belt 108 contacting the fixing roller 103 over a specified angular range such that a contact portion is formed and a pressure roller 107 is disposed inside the heat resistant belt 108 at the exit point of the contact portion, and with this device, distortion is caused in the elastic body 102 of the thermal fixing roller 103 by making the pressure roller 107 bear against the thermal fixing roller 103 under pressure through the heat resistant belt 108.
When experimenting with the construction of this belt contact fixing device and carrying out a fixing test by varying the load value of the pressure roller 107, the present inventors found a new image defect. If the load of the pressure roller 107 is more than a specified value, the toner image 101 fixed on the transferred medium 100 is displaced.
FIG. 8 shows the relation between the image displacement and the load value of the pressure roller as found by experiment.
The mechanism proposed for the image displacement phenomenon investigated by the present inventors is now described.
Generally, as shown in FIG. 9, if a pressure roller 121 is made to contact the surface of the elastic roller 120 under pressure, the surface of the elastic roller 120 is deformed by the contact pressure from the pressure roller 121. If distortion is caused in the surface of the elastic roller 120, the circumference thereof is increased by a corresponding amount. The circumference L before the distortion is caused, wherein r is the radius of the elastic roller 120, is: EQU L=2 .pi.r
and the circumference L' after the distortion is caused, wherein s is the amount of distortion is:
L'=2 .pi.r (1+.epsilon.)
Accordingly, the amount of distortion is: EQU .epsilon.=(L'-L) / L=(L' / L)-1
The surface speed of the elastic roller 120 is influenced by the distortion .epsilon., and if .epsilon. is caused in the direction around the circumference of the elastic body, the surface speed V in the portion where .epsilon. is caused is: EQU V=(1+.epsilon.)V.sub.0
Here, V.sub.0 is the surface speed without distortion .epsilon., or in other words when .epsilon.=0.
With the belt contact fixing device according to the above described proposal, distortion is caused in the direction around the circumference by the load from the pressure roller 107 at the exit point, and there is a small difference between the surface speed V in the portion where the distortion is caused and the surface speed V.sub.0 at the point where only the belt 108 contacts the fixing roller and having a value of .epsilon. of 0. If there is such a difference between the surface speed of the fixing roller 107 at the initial contact portion and that at the exit point of the belt contact portion, and the difference is greater than a specified value, the recording medium so far attached to the surface of the fixing roller cannot cope with the difference and is transported with the surface speed of the deformed portion, wherein the pressure roller 107 having a high contact pressure, that is a high transporting force, contacts the fixing roller, and thus image displacement is considered to be caused.
The amount of distortion on the surface of an elastic roller, which is caused by a specified load value, can be calculated. If a hard roller and a soft roller contact each other under pressure with a certain load value, the surface of the soft roller is elastically deformed in the contact portion, and distortion e is caused in the surface in the rotating direction of the roller. If a recording medium is passed through the contact portion by rotating the pair of rollers, the recording medium is transported through the deformed contact portion, which makes the length L', the transported length of the recording medium by a single rotation of the deformed elastic roller, longer than the original circumference L by an amount corresponding to .epsilon..
Here, the distortion .epsilon. is:
.epsilon.=(the length L of a recording medium, which is transported by a single rotation of an elastic roller / the circumference, when .epsilon.=0, of an elastic roller) -1
The actual distortion .epsilon. can be calculated by this formula, and values of distortion .epsilon. shown in this specification are calculated this way. When measuring the distortion .epsilon. using an actual belt contact fuser, it is necessary for the belt tension to be zero.